debbie herzman is chairman of ntsb. >> we've learned from accidents, plowed those lessons back, in they've been embraced and we're not seeing repeats of those same accidents. >> reporter: experts say two accidents in which human failure was at least partly to blame underscore that remains the biggest threat to safety. air france 447 crashed into the atlantic in 2009, killing 228. that same year, regional airline colgan air flight 447 crashed in buffalo, killing 50. >> colgan air was a watershed event for aviation safety, in particular, bringing the regional carriers up to standards with the major carriers. >> reporter: the safety culture is spreading. accident rates are dropping in russia, africa, latin america, the caribbean, and asia. despite the dramatic improvement experts say there's danger here. just when you start to think, it can't happen again, is often when you're the most vulnerable. tom cocostello, nbc news, washington. >>> stocks provided many 401(k)s, pensions, and other investors pretty good news last year. despite the bad economic news. traders get the day off but the dow w